Because it deals with the dead and evil spirits. People always tend to view evil as having to do with the devil/Satan. Then include with that belief the symbols of the black cat, witches, bats and spiders, all symbols of evil or bad things, and the stories of the dead roaming the earth, etc., you have a bunch of people thinking that Halloween is bad.. Nevermind the fact that it was started as a celebration of the end of the harvest and the beginning of winter, nor that the Halloween that we observe was actually started by the "Church" to honor saints and to cleanse the spirits of the dead so they may go into Heaven.